Composite solid rocket propellants consist of a rubbery matrix called a binder in which particles of solid oxidizing compounds are embedded. In addition to the oxidizer, the particulate solids of the propellant may include fuel elements, ballistic modifiers and/or other special-purpose solids. The binder consists of an elastomer which may or may not be plasticized with energetic or non-energetic dissolved liquids, and may contain other special-purpose dissolved liquid additives to impart particular ballistic or physical properties to the propellant.
Prior to the present invention, elastic composite propellants have derived their structural properties from elastomers which are chemically cross-linked. To prepare such a propellant, it is necessary to start with a liquid precursor of the elastomer, usually an oligomer in the 500-3000 average molecular weight range, in order to have the fluidity required for incorporating the other ingredients. After thoroughly mixing into this precursor all the other ingredients of the propellant, a curing agent is added which chemically reacts with the oligomer to convert it to an elastomer via chain extension and cross-linking. All processing and testing requiring propellant flow subsequent to addition of the curing agent, such as characterization tests and casting into rocket motors, must be accomplished in the period of time before the cure reaction renders the mixture unmanageably viscous. This period of time is termed the pot life. It is common in the industry that pot life strongly influences processing parameters, with a resulting impact on cost.
Once the binder of a composite propellant is cross-linked via the cure reaction, the propellant is very difficult to dispose of except by burning. Many military rockets reach obsolescence and require disposal of their propellant. Burning as a means of disposal is undesirable for environmental reasons as well as for the waste of materials which results.
It is apparent from the foregoing discussion that many problems associated with state-of-the-art composite propellants could be eliminated if the elastomeric properties of the binder did not require chemical cross-linking, but depended rather upon a thermally reversible physical phenomenon such as melting and crystallizing. Elastomers with this type of behavior have been available in recent years, known by such terms as thermoplastic elastomers. On the molecular level, such elastomers consist of hard segments, which are usually crystalline, and soft segments which are amorphous and which impart the rubbery properties of the material. Typical of such thermoplastic elastomers are block copolymers of monomers such as styrene and a diene, where the styrene blocks form the hard segments and the diene blocks form the soft or rubbery segments. There are various other types of thermoplastic elastomers as well.
The concept of utilizing thermoplastic elastomers as binders for composite propellants has been considered by the propulsion industry for many years. This is evidenced by the fact that virtually all new elastomers are considered as potential propellant binders as soon as they become known to the propulsion industry.
The approach of using thermoplastic elastomers for propellant binders has been centered around the conventional processing techniques which require processing by adding solids to the fluid fractions. However, in the course of attempts at using thermoplastic elastomers for binder ingredients by standard state-of-art processing techniques, artisans have concluded that it would be impractical if not impossible to mix solid particulates at the levels of interest into most thermoplastic elastomers while they are held above their melting points.
An object of this invention is to make the desires of the propellant industry become a reality by providing the combinations of techniques and formulations which enables thermoplastic elastomers to be utilized as the binders for composite propellants.
A further object of this invention is to overcome the obstacles of processing thermoplastic elastomers by providing a technique which employs the combination of thermoplastic elastomers in solution by common volatile organic solvents while processing.
Still a further object of this invention is to provide the technique of mixing the particulate solids of a composite propellant into a solution of a thermoplastic elastomer which technique overcomes the obstacles of the prior art processing technique while offering many advantages over the processing of composite propellants by conventional prior art techniques.